Commercially-available piston-based ventilators operate to deliver a preset tidal volume at a preset pattern and rate of flow. Several models are available which satisfactorily perform this function. With such ventilators, however, the patient cannot independently influence the flow rate or volume once a cycle is triggered. In certain applications, it is more desirable to utilize ventilators with which the controlled variable is pressure, as opposed to volume or flow, output. Here, the control system of the ventilator is designed to alter pressure according to specified functions. Although this approach does not guarantee a specific tidal volume, it does allow the patient the freedom to influence his/her breathing pattern during the supported breath. This results in greater comfort.
Prior art exists for piston-based ventilator designs which are intended to deliver pressure according to specified functions (e.g. U.S. Pat. No. 4,448,192 to Stawitcke, Younes, J. Applied Physiology 62:2491-2499, 1987). In all these prior art designs, pressure is controlled according to a servo feedback mechanism in which the actual pressure is compared with the desired pressure, an error signal is generated, and this error signal is used to drive the motor which, in turn, alters the pressure in the piston, thereby minimizing the error signal and maintaining the actual pressure as close as possible to the desired function. This approach, however, has two major disadvantages:
1. By definition, an error (difference between actual and desired pressure) must exist in order for the motor to respond. This error theoretically can be minimized by increasing the gain of the error signal. However, because in ventilated patients, pressure in the tubing is invariably highly variable, and because of obligate delays between the generation or change in error signal and the actual change in pressure, a high error gain would be associated with wide fluctuations in the command signal to the motor and hence oscillations. Thus, if the error gain is set such that a difference in pressure of 1 cm H.sub.2 O between actual and desired pressure results in a piston pressure output of 30 cm H.sub.2 O, a relatively small variation in the actual pressure signal (e.g. .+-.1 cm H.sub.2 O) would initiate oscillations of considerable amplitudes. This consideration makes it necessary to use relatively low error gain or substantial filtering techniques, both of which result in poor responsiveness of the device. Thus, if a patient pulls harder, which tends to reduce system pressure, the compensation by the device will be slow and incomplete, thereby causing actual pressure to deviate substantially from desired pressure.
2. This type of control makes it progressively more difficult to regulate adequately pressure at points more distal from the piston and, hence, at points closer to the patient. This is because all tubing used to connect piston to patient's airway displays substantial compliance and resistance. There are, therefore, obligate delays in pressure transmission from piston to patient. The farther out (from the piston) the point used for pressure feedback (used to generate the error signal) is located, the greater the delay between a change in piston pressure in response to a change in error signal and those changes being detected at the site used for pressure feedback. This delay would tend to result in the motor overcompensating with a pressure overshoot which, once the overshoot reaches the pressure monitoring site, will unnecessarily reduce the error signal resulting in an undershoot. A tendency for oscillation is again created which is greater the closer the pressure feedback site is to the patient's airway, and also greater the higher the compliance and resistance of the tubing between piston and pressure monitoring site. To offset this instability, the response must be damped with more damping required as the site for pressure control is advanced closer to the patient. Since a more damped control system responds slowly, a conflict, therefore, arises between the need to control pressure as close to the patient as possible and the responsiveness of the ventilator to changes in airway pressure produced, for example, by changes in patient demand.
One aspect of this invention is directed towards solving the servo-feedback problem by employing what is termed herein a "piecemeal" approach to piston control.
All commercially-available piston-based ventilators are currently utilized to deliver predetermined tidal volumes at predetermined flow rates. In other words, they function as volume ventilators. In recent years, there has been a trend away from this type of ventilation to other modalities in which the ventilator delivers pressure at the patient airway according to desired functions. Neither flow nor volume is predetermined as the patient determines these through changes in his own effort. This type of ventilatory support is more comfortable and associated with less risk to the patient. Examples of this type of approach include pressure support ventilation (PSV), in which the ventilator controls pressure at the airway according to specified and predetermined functions of inspiratory time, and the more recent Proportional Assist Ventilation (PAV) as described in U.S. Pat. No. 5,044,362 and in more detail in published European patent application No. 452001 and Younes et al, American Review of Respiratory Diseases, col. 145, pp 114-129, 1992, the disclosures of which are incorporated herein by reference in which delivered pressure is a function of ongoing inspired volume and rate of flow of gas from ventilator to patient. With any of these newer approaches, the patient must be able to freely alter the rate of gas flow while the machine maintains pressure according to the desired function regardless of patient effort, and hence flow rate. At present, these options are commercially-available only on ventilators operated with pneumatic valves which control flow from a high pressure source (e.g. Puritan-Bennett 7200 and Siemens-Elema 900C) and in blower-based ventilators (e.g. Respironic's BiPAP). It would be advantageous to be able to use piston-based ventilators to deliver these newer pressure support options.
In commercially-available piston-based ventilators the chamber is sealed through physical contact between the rim of the piston and inner surface of the cylinder. This design is unsuitable for the delivery of these new pressure support options. This is because the friction between piston and cylinder wall is not only high but non-uniform. The degree of friction varies from one position to the next along the cylinder and from time to time as a result of wear. This variable resistance results in variable dissipation of the force generated by the motor and, hence, variability in the relation between desired and obtained pressures. Servo-feedback using airway pressure can be used to optimize this relation despite the variable friction. However, as indicated earlier, there are serious limitations to the use of the airway pressure feedback to servo-control ventilator pressure output in order to implement these pressure support modalities. For adequate performance in this regard, the friction between piston and cylinder must be minimal.
Rolling seal pistons can be designed such that the friction is negligible. Such pistons have been utilized in prior art devices intended to deliver pressure according to desired functions (e.g. Stawitcke U.S. Pat. No. 4,448,192; Younes et al U.S. Pat. No. 5,044,362. Although rolling seal pistons offer satisfactory performance for the purpose of delivering pressure support options, they have two drawbacks. First, the survival of the sealing diaphragm is influenced by many hardware design features and operating conditions. There is always the risk of unexpected rupture with potentially catastrophic results. Second, with piston ventilators, the gas intake is often not from a pressurized source but simply from the room. The piston must be forcefully retracted in the exhalation phase to refill the chamber with fresh gas. The negative pressure thus created often causes inversion or double convolution of the diaphragm which in turn causes a variable increase in resistance and marked reduction in diaphragm survival.
Another aspect of the present invention is directed towards solving the piston friction problem by designing a piston which has a very low resistance to movement in the piston chamber.